1. Field of the Invention
The invention described herein relates to a process for forming surfactants for use in detergent compositions where a step in a process includes cooling the reaction mass, following the mixture of an alkaline component, the detergent acids and excess sulfating agent.
2. Discussion of the Art
The use of anionic surfactants particularly those where the anionic character is caused by a sulfonate or a sulfate group is well known in the detergency arts. Further, the sulfation or sulfonation of precursor materials such as alkylbenzene to form alkylbenzene sulfonic acid which is subsequently neutralized to the sulfonate is also well known in the art. For instance, U.S. Pat. No. 3,024,258, issued to Brooks et al, Mar. 6, 1962, discloses a process for sulfonating a reactant continuously and rapidly as well as for separating the resulting sulfonated reactant from the excess sulfonating agent and to the continuous neutralization of the resulting detergent acids. During the neutralization step the Brooks et al patent describes cycling the neutralized product through a heat exchanger to maintain the temperature in the range of from 85.degree. F. to 140.degree. F. The examples of Brooks et al indicate that the final product contains sodium sulfate in water in a ratio of from about 1:11. The Brooks et al patent is herein incorpated by reference.
Similarly, other patents describing sulfonation and sulfation processes are U.S. Pat. No. 3,259,645 issued July 5, 1966, U.S. Pat. No. 3,363,994 issued Jan. 16, 1968, U.S. Pat. No. 3,350,428 issued Oct. 31, 1967, and U.S. Pat. No. 3,427, 342 issued Feb. 11, 1969, all to Brooks et al which are herein incorporated by reference. Earlier patents describing sulfonation processes include U.S. Pat. No. 2,129,826, Reilly issued Sept. 13, 1938 and U.S. Pat. No. 2,039,989, issued to Gressner May 5, 1936, both of which are herein incorporated by reference.
In the process of forming anionic surfactants which have a sulfuric or sulfonic acid moiety it is necessary to react a precursor with a sulfating agent which is a material such as sulfur trioxide to form the organic sulfuric or sulfonic acid. Materials which supply a source of sulfur trioxide for the forming of such detergent acids are known as sulfating agents and the term embraces sulfonating agents as well. Sulfating agents include pure sulfur trioxide or sulfur trioxide diluted with a gas which is inert in the reaction, such as hydrogen chloride or sulfur dioxide. The most common sulfating agent, however, is oleum which is a mixture of sulfur trioxide dissolved or suspended in sulfuric acid. The method of formation of the detergent acids, also known as the acid mix, is not material to the present invention up to the point that an excess of the sulfating agent should be present in addition to that which is required to react the detergent precursor to the desired degree of sulfation.
The reason for using an excess of the sulfating agent is basically to ensure that the detergent precursor which is a relatively expensive material will be completely reacted. That is, for ecological, product performance and cost reasons, it is undesirable to leave unreacted alkylbenzene in the detergent product as it is relatively volatile and will in the instance of spray-dried formulations be driven off upon heating.
The step following the reaction of the detergent precursor and the sulfating agent is that of neutralizing the mixture containing the organic sulfuric or sulfonic acid. This mixture will also contain the excess sulfating agent, and water which is either introduced with the reactants or formed during the sulfation reaction. This mixture is then neutralized with an alkaline component such as sodium hydroxide or sodium carbonate or a similar material to form the sodium salt of the organic sulfuric or sulfonic acid. The introduction of the alkaline component, however, also neutralizes the excess sulfating agent to form sodium sulfate.
This second mixture referred to herein as the reaction mass then contains the sodium salt of the organic sulfuric or sulfonic acid, sodium sulfate, water, and small amounts of the excess alkaline component. As the sulfation reaction and the neutralization reaction are both highly exothermic it is necessary to quench the heat of reaction to avoid bringing the reaction mass to boil as well as to avoid undesirable secondary reactions which may take place. The most common method of quenching any exothermic reaction is to pass the product of the reaction through one or more heat exchangers where excess thermal energy is removed thus lowering the temperature of the product for further processing. It is noted, that the sulfation reaction mixture may be quenched through heat exchange prior to the neutralization reaction if desired although the present invention only relates to heat exchange following the neutralization step.
The most commonly used heat exchangers for the preparation of detergents are simply a large conduit through which the reaction mass passes and a series of smaller conduits within the larger conduit through which the cooling medium flows. In operation the cooling medium is of course maintained at a temperature below that of the reaction mass which swirls around the smaller conduits. The thermal energy then flows through the walls of the smaller conduits where the heat energy is transferred to the cooling medium and removed from the system. Thus, the reaction mass is cooled to a desirable temperature for further processing.
Known systems for the neutralization step have involved processing the reaction mass in diluted form in the presence of large volumes of water. The water is present in the reaction mass from the neutralization and from the alkaline component, e.g., a solution of caustic. Water may also have been added directly to the reaction mass to purposely dilute the heat generated by the reaction.
Obvious economic reasons dictate that the presence of a large volume of water in the reaction mass is undesirable. For instance, the water present in the reaction mass must be removed if the end product is to be solid such as a spray-dried granule. Moreover, the presence of the water in the reaction mass requires that storage or processing facilities have greater volume than that required for a reaction mass with lower water content. Conversely, lowered water content in the reaction mass allows greater throughput of the final product with existing equipment.
It is also observed, aside from the advantages listed above, that other processing goals can be achieved by lowering the water content of the reaction mass. For example, sodium sulfate in dry form is usually added to the reaction mass following the heat exchange operation to aid in the preparation of granular detergent compositions. If desired, however, in the present invention the sodium sulfate may be formed in situ during the neutralization step by using excess sulfating agent over that which is needed to accomplish sulfation of the organic precursor. The excess sulfating agent is then neutralized by the alkaline component to form sodium sulfate. In the case where oleum is used as the sulfating agent versus sodium sulfate to generate a source of sodium sulfate in the product a density and cost/availability advantage favor the use of oleum. Cost and availability is of course a readily apparent advantage while the density factor allows equivalent storage facilities to hold a greater weight of oleum as opposed to dry sodium sulfate.
An additional advantage to lowering the water content of the reaction mass resides in the difference of incorporating wet versus dry silicates into the detergent compositions. Most detergent products require the presence of alkali metal silicates to provide an anti-corrosion benefit to exposed washing machine surfaces as well as to provide non-gooey granules, e.g., granules which cake or do not flow freely under humid conditions.
The silicates, as stated above, may be added to the crutcher mix containing the reaction mass as a wet or dry material. If the water content of the crutcher mix is low, as is obtained in the present invention, then a slurry of wet silicate may be added to the crutcher mix. If the water content of the crutcher is already high from the aqueous reaction mass, then it is usually necessary to add dry silicate to reduce the crutcher water content to lower the drying load when forming the crutcher mix into granules. Drying load as used above is defined as the heat energy required to remove water in granules formation. It is also observed that not withstanding the use of costly energy for drying the crutcher mix, that a point can be reached where the crutcher mix is too wet to be dried by conventional spray-drying towers such as those described in U.S. Pat. Nos. 3,629,951 and 3,629,955, both issued to R. P. Davis et al on Dec. 28, 1971, which are herein incorporated by reference.
It is thus seen that reducing the water content of the reaction mass and subsequently that of the crutcher mix is highly desirable. To effectively reduce the water content of the reaction mass it is necessary that the sodium sulfate be supersatured in relation to the water. This is not undesirable as the sodium sulfate cannot be economically removed in a continuous detergent making operation and in any event the sodium sulfate is a very desirable ingredient, especially in its ability to act as a structurant to avoid gooey granules as previously stated in the discussion concerning the function of the silicate.
It has been observed, however, that when the reaction mass is passed through a heat exchanger with the sodium sulfate in a supersaturated condition that the heat exchanger immediately suffers a reduction in heat energy transfer capacity.
This loss of energy transfer capacity has been determined to be caused by the buildup of anhydrous sodium sulfate in the heat exchanger. Moreover, the loss of energy transfer capacity continues until the heat exchanger is completely plugged with the reaction mass. Thus, while it is extremely desirable to operate the detergent making process under conditions where the sodium sulfate is supersaturated in the reaction mass it has been impractical, if not effectively impossible, to do so.
The difficulty which the present invention alleviates is caused by the sodium sulfate which when supersaturated in the aqueous reaction mass precipitates on the surfaces of the smaller conduits in the heat exchanger and continues to precipitate until the entire heat exchanger is plugged with the precipitated sodium sulfate. At this point if there is but a single heat exchanger the neutralization reaction as well as the earlier sulfation reaction must be shutdown and the heat exchanger torn apart and cleaned or flushed with water to remove the precipitated sodium sulfate.
Alternatively, the sulfation reaction can be allowed to continue to proceed along with the neutralization reaction, however, additional capital expense is then necessary to provide a parallel series of heat exchangers through which the neutralized reaction mass is allowed to pass while the first heat exchanger has the sodium sulfate removed. Either alternative is quite costly and extremely undesirable.
A second alternative is to process the reaction mass with sufficient water present so that the sodium sulfate never becomes saturated in the reaction mass. However, such processing requires large amounts of water which, as previously discussed, is undesirable.
In view of the high degree of interest of operating heat exchangers at high capacity when removing heat from a neutralized detergent acid mix the following objects of the present invention are developed.
It is an object of the present invention to provide a method for rapidly and economically removing heat from a neutralized detergent acid mix.
It is a further object of the present invention to prepare an aqueous mixture of supersaturated sodium sulfate and the sodium salt of an organic sulfuric or sulfonic acid having as a processing step the cooling of the mixture in a heat exchanger while introducing a slurry of anhydrous sodium sulfate into the reaction mass to reduce the deposition of sodium sulfate in the heat exchanger.
Throughout the specification and claims, percentages and ratios are by weight and temperatures are in degrees Centrigrade unless otherwise indicated.